This book addresses two issues: why the US uses force, and why some leftists and liberals provide support. Both give rise to speculative responses, and propel the writer into a hazardous exercise. Who can say, after all, the real reason that President Bush decided to go to war in Iraq, or what truly motivated a particular individual to lend support. Beyond instinct or intuition, both issues require mastery of many factors - history, geopolitics, money, psychology, political philosophy, to suggest but a few. To engage in both tasks, as Richard Seymour does with this ambitious book, is to undertake a project that faces considerable hurdles.

Seymour believes that the US has long been engaged in an imperial enterprise, and that its foot soldiers include a great number of liberals and progressives (Nick Cohen and Michael Ignatieff among them). Seymour casts these thinkers and writers as enablers. "Imperialism is not a distant relic, but a living reality," he writes, "and the moralisation of the means of violence has been the task of liberal and progressive intellectuals since they first competed with clerics for moral authority." The charge is deep. It may be sustainable for some of his targets, in some instances, but the generality of the attack undermines its effectiveness.

One reason is that Seymour never grapples with the reality that the US has used force for a multitude of different reasons over the past five decades, and that some instances are justifiable whereas others are not. In short, not every use of force justifies a charge of murder. You need some basic criteria to distinguish between what is just and unjust, lawful and unlawful, murderous or not. Seymour doesn't identify any. On this account, it seems all force is wrong, so that any liberal support may be treated as liberal justification for murder. That doesn't hold up. Iraq I (1990) wasn't Kosovo, which wasn't Sierra Leone, which wasn't Afghanistan, which wasn't Iraq II (2003). There is no seamless link between these military expeditions. The reality is more complex, and requires engagement with a basic question: when can one state use military force against another?

The answer, in law if not morality, was "settled" by the UN charter in 1945, a document that Seymour ignores. Against the backdrop of the second world war, the then world of nation states - less numerous than today, ostensibly more colonial - came together to replace the status quo that basically allowed them to use force whenever they wanted. Under the new rules, military force was lawful in just two circumstances: self-defence (when an armed attack had occurred or was imminent) or where the security council authorised its use (requiring a resolution adopted by a positive vote with no permanent member voting against). That, at least, was the theory. In practice, the scheme had many opponents, not least the neocons who sat in the upper galleries of the Bush administration, drawing inspiration from another bunch of former progressives - men such as Irving Kristol and Nathan Glazer - from the 1950s who proceeded to travel another path.

The UN drafters sowed the seeds for a third possible justification for war, the heart of Seymour's critique. For the first time in a multilateral treaty, the charter gave legal force to the notion of fundamental human rights for all. But that commitment was to be balanced with an obligation not to interfere in the domestic affairs of another state. Ever since, the $64m issue has been how to balance these competing commitments. Did the drafters of the charter envisage circumstances in which a huge threat to human rights in one country could justify the use of force? An affirmative answer opens the door to humanitarian intervention. Seymour seems to come down on the side of those who believe human rights violations should not justify force, while many of those he aims at - irrespective of whether or why they supported some or all of Iraq I and II, or Kosovo, or Afghanistan (in 2001) - take the opposite view.

So the book becomes a bit of a rant. In charting the intellectual roots of this apparently open-ended appetite for violence, mayhem and murder, important points of detail are missed - what was the justification for the war? - and the transformed framework of rules and principles is bypassed. Iraq I was explicitly authorised by security council resolution, Iraq II was not. Afghanistan was, at least initially, seen to be justified by the unanimous security council resolution 1373, an act of self-defence. Many will not be pleased by such security council actions, but their existence has important consequences and they cannot be ignored.

Humanitarian intervention has been the subject of longstanding attempts at codification. After Kosovo, which was problematic on many grounds, the Canadian government sponsored an effort to develop new principles, known as the Responsibility to Protect. After 2003, that effort ground to a halt, as Iraq made clear the potential for abuse. Over the long term, the real critique of those who supported the latest Iraq war is that they killed off any hope, for now at least, of garnering support to use force where massive violations of fundamental human rights are taking place. It is not sufficient to label the US as "the chief inheritor of the legacy of violent white supremacy". The more obvious conclusion - if such a claim is to be made - is that those who are on the receiving end of what Seymour perceives as US excess have, through the acts of their own governments, or their failure to object, contributed to their own oppression.

The Liberal Defence of Murder glosses over vastly important issues. Was the post-second world war human rights project intended to create new conditions of colonial domination? Has it contributed to circumstances in which there will be more oppression and misery, rather than less? Have the economic rules promoting globalisation engendered war? A scattershot aim at "liberal and progressive intellectuals" doesn't hit home. Force can be justifiable in some circumstances, in domestic law and in international law. The difficult issue is when, and the answer to that turns on the particularities of each case. The generality of Seymour's conclusion, the broad sweep of his argument and the passion of his attack are overstated, dissipating their force. More nuance and context could have made this potentially important book compelling. It is a shame, as buried in these pages and their footnotes is a great deal of damning material on the apologists of recent illegalities.

• Philippe Sands's Torture Team: Uncovering War Crimes in the Land of the Free is published by Allen Lane.